media: mtk-vcodec: move firmware implementations into their own files

mtk-vcodec supports two kinds of firmware, VPU and SCP. Both were
supported from the same source files, but this is clearly unclean and
makes it more difficult to disable support for one or the other.

Move these implementations into their own file, after adding the
necessary private interfaces.

[hverkuil: smatch fix: mtk_vcodec_fw_vpu_init() error: uninitialized symbol 'rst_id'.]

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Fixes: bf1d556a

 ("media: mtk-vcodec: abstract firmware interface")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>